Check valves of the prior art have included plates which were pivotally mounted within flow passage and spring loaded toward closed position. Examples of this type of structure are shown in the J.S. Wheeler, Jr. U.S. Pat. Nos. 3,007,488; 3,026,901; 3,072,141; and 3,074,427. These valves are closed by spring action. Thus, they prevent all flow in one direction and greatly restrict low flows in the opposite direction but open fully with increased flows.